The present invention relates to a surface inspection method and a surface inspection device for detecting micro-size flaws or foreign objects attached on a surface of a substrate such as a semiconductor wafer or the like.
In the past, in case that micro-size foreign objects attached on a surface or micro-size flaws on a surface are to be detected, detection of the foreign objects or the flaws have been carried out by projecting a laser beam to a surface to be inspected and by receiving and detecting a scattering light from the foreign objects or the flaws.
FIG. 8 shows general features of a surface inspection device. As an object to be inspected, a silicon wafer 1 is shown.
The silicon wafer 1 is held at horizontal position by a substrate chuck 2, and the substrate chuck 2 is rotated at a predetermined rotation speed by a motor 3.
An inspection light projecting system 5 and a scattering light receiving system 6 are positioned against an inspection surface 4 of the silicon wafer 1. The inspection light projecting system 5 projects an inspection light from a light source unit to the inspection surface 4. For instance, a laser beam 7 is projected at an incident angle as required, and a reflected light 7′ of the laser beam 7 reflected by the inspection surface 4 is detected by a reflection light detector 8. Light intensity of the reflected light 7′ of the laser beam 7 detected at the reflection light detector 8 is sent by feedback in order to keep the projection intensity of the laser beam 7 at a constant level.
The scattering light receiving system 6 has an optical axis, which crosses an optical axis of the inspection light projecting system 5. On the optical axis, a lateral scattering light detector 9 and a forward scattering light detector 11 are arranged. The lateral scattering light detector 9 and the forward scattering light detector 11 have different crossing directions and different crossing angles. The lateral scattering light detector 9 and the forward scattering light detector 11 detect the scattering light occurred by a foreign object or a flaw when the laser beam 7 is projected and issue electric signals to match the received light amount.
For the inspection of the inspection surface 4, the inspection surface is rotated by the motor 3 under condition that the laser beam 7 is projected on the inspection surface 4. Further, the inspection surface 4 is shifted in radial direction of the silicon wafer 1 at a predetermined pitch (a predetermined speed). The projecting point of the laser beam 7 is shifted in radial direction while rotating, and the laser beam 7 scans over the entire surface of the inspection surface 4. When the laser beam 7 passes through the foreign objects or the flaws, the reflected light is scattered, and the lateral scattering light detector 9 and the forward scattering light detector 11 receive a scattering light 12 respectively.
The scattering light 12 thus received is converted to an electric signal by a photoelectric conversion element. Further, the electric signal is amplified by an amplifier and is processed as a foreign object signal and is stored in a storage unit as a detection result.
The examples of output signals of the lateral scattering light detector 9 and the forward scattering light detector 11 are shown in FIG. 9. Normally, the output signal contains a signal component S and a direct current component D. In case that the inspection surface 4 has a smooth surface as it is polished well or the like, and there is little scattering reflection on the inspection surface 4 itself, the direct current component D is low. In case that the inspection surface 4 has rough surface and there is much scattering reflection on the inspection surface 4, the direct current component D is appeared highly.
Next, FIG. 10 is a diagram to show a relation between the detection light intensity and the signal output level when the lateral scattering light detector 9 and the forward scattering light detector 11 receive the light. Normally, the inspection light intensity and the signal output level are in a proportional relation as shown by a curve A in FIG. 10. When the inspection light intensity exceeds a predetermined value and the signal output level reaches a proportional limit (a measurement limit), the output signal comes to lose linear and is turned to saturated state. Therefore, a range I where the proportional relation is maintained, is a measurement range (a dynamic range) of the detector.
However, when the inspection surface 4 has rough surface (e.g. rear surface of the wafer), or when the inspection surface 4 is a wafer surface with a metal film formed on the wafer surface, irregular reflection on the surface itself is increased. As a result, the direct current component D as shown in FIG. 9 increases and the detection light intensity may exceed the measurement range. Then it is substantially impossible to perform the measurement.
To solve this problem, as a method to extend the measurement range, there is a method to logarithmically amplify the signal by using a logarithmic amplifier. In case that the signal is amplified by using the logarithmic amplifier, the signal output level is shown as a curve B in FIG. 10.
In case that the logarithmic amplification is performed, the more the detection light intensity increases, the less the increasing ratio of the signal level is. As the comparison between the curve A and the curve B shows, when the detection light intensity reaches at the measurement limit, the value of the detection light intensity increases by IA. That is, the measurement range is extended by IA.
However, when logarithmic amplification is performed, the output signal with the detection light intensity of low level is emphasized. For example, in case that the light intensity is P, the signal output level is higher in the curve B by ΔS compared with the curve A. Accordingly, the noise of low level is detected higher, and the S/N ratio tends to be decreased. In case that the detection signal range is in high frequency band, higher S/N ratio and higher responsiveness are required on the logarithmic amplifier. Therefore, the logarithmic amplifier becomes expensive.
A surface inspection device is described in JP-A-2004-217519. The device to extend the dynamic range by logarithmic amplification is described in: JP-A-2005-526239, and “New Optical Microscope (Vol. 1); Laser Microscope; Theory and Practice”; Supervised by Tetsuya FUJITA, edited by Satoshi KAWATA; Gakusai Kikaku Co., Ltd.; Mar. 28, 1995; p. 116.